Thermal fuse with triggering mechanism



Nov. 19, 1968 A. o. VAN HORSSEN 3,412,356

THERMAL FUSE WITH TRIGGERING MECHANISM Filed March 21, 1967 Om hw jma wJQmDu Nm INVENTOR. ARDEN D. VAN HORSSEN atwmgv/mowwm Nn o o m wmQw 2 8 5 32 592 9 on mm [3 ATTORNEYS United States Patent 3,412,356 THERMAL FUSE WITH TRIGGERING MECHANISM Arden D. Van Horssen, Minneapolis, Minn., assignor,

by mesne assignments, to United-Carr Incorporated,

Boston, Mass., a corporation of Delaware Filed Mar. 21, 1967, Ser. No. 624,951 5 Claims. (Cl. 337-150) ABSTRACT OF THE DISCLOSURE A plunger is urged against a fusible pellet by a plurality of resilient tines or spring fingers. When the pellet melts, the plunger then moves and the free ends of the tines flex inwardly out of obstructive engagement with a spring biased sleeve member. The sleeve member, the release of which has now been triggered, moves under the influence of its spring to open the contacts, one contact being attached directly to the sleeve member so that it moves in unison therewith.

BACKGROUND OF THE INVENTION Field of the invention This invention relates generally to circuit breaking devices, and pertains more particularly to a thermal fuse which interrupts or opens an electric circuit when a predetermined ambient temperature is reached.

Description of the prior art Thermal fuses, of course, are not new. However, prior art devices of this type have certain shortcomings. One notable shortcoming resides in the lack of a triggered release which allows the engaged contaots to quickly separate when the fusible element or pellet has become sufliciently molten. Also, other devices of this general character do not lend themselves readily to miniaturization, and in this regard, some of them are difficult to assemble. Still further, the prior art devices in many instances are complex and relatively expensive to manufacture. These problems, while individually solved in some prior art devices, leave a need for a device that has all the attributes listed above and which are incorporated in a thermal fuse constructed in accordance with the teachings of the present invention.

SUMMARY OF THE INVENTION Briefly stated, my invention involves a trigger mechanism which becomes effective to allow quick disengagement of normally engaged contacts when a fusible pellet or element has been subjected to a sufficiently high temperature which renders it molten. One contact is attached to a sleeve or cup-shaped member which is normally prevented from moving in one longitudinal direction under the influence of outwardly urged resilient tines or spring fingers which are then in obstructive relationship with the sleeve or cup-shaped member. The free ends of the tines are formed with inclined cam surfaces that coact with a similarly inclined surface formed on one end of a longitudinally shiftable plunger. The other end of the plunger bears directly against the fusible pellet. Consequently, when the pellet is in a solid condition, the tines merely urge the plunger against the pellet, but when the pellet becomes molten, then the plunger is free to move in that direction which effects the release of the outwardly urged tines so that they move inwardly to allow the sleeve or cup-shaped member to move rapidly under the influence of a compressed coil spring which then expands. Since one contact is attached to the sleeve or "Ice cup-shaped member, it is immediately separated from the fixed contact with which it cooperates.

BRIEF DESCRIPTION OF THE DRAWING FIGURE 1 is a longitudinal sectional view through the center of a thermal fuse embodying my invention, the telescopically arranged contacts thereof being illustrated in an engaged relationship prior to the fusible pellet becoming molten, and

FIGURE 2 is a view similar to FIGURE 1 but showing the contacts disengaged, this being after the fusible pellet has become molten.

DESCRIPTION OF THE PREFERRED EMBODIMENT The thermal fuse illustrating my invention comprises a metallic casing 10, which can be less than an inch in length, having a socket 12 at its closed end into which is soldered a wire 14. The casing 10 has a cylindrical bore 16 extending from its closed end to its other end but adjacent its other end is a counterbore 18 of slightly larger diameter than the main bore 16 which forms an internal annular shoulder at 20.

A metallic bushing 22 having an outturned end 24 and an inturned fiange 26 with a central opening 28 is pressfitted in the counterbore 18. A plurality of notches 30 formed in the end portion of the casing 10 permits solder 32 to assure that the bushing 22 will be held in place.

Within the bushing 22 is an insulating sleeve 34 of suitable dielectric material. The sleeve 34 encircles a tubular contact 36, the contact 36 being retained in the sleeve 34 in a fixed relation therewith. The tubular contact 36 forms a socket for the second wire 38, the wire 38 being soldered in the tubular contact 36 in the same fashion as the wire 14 is soldered in the socket 12. A beveled entrance 40 is formed at the end of the tubular contact 36 opposite the wire 38.

The beveled entrance 40 facilitates the telescopic insertion of a cooperable contact 42, the contact 42 being in the form of a split ball as can readily be discerned from the drawing. Integral with the bulbous contact 42 is a shank 44 that has a slot 46 therein which slot actually forms the split of the ball contact 42. The shank 44 is in turn integral with a spring guide 48 which has attached thereto a sleeve or cup-shaped member 50. The spring guide 48 is of lesser diameter than the sleeve 50 so as to form a shoulder at 52. The sleeve 50 has an internal bore 54 which can be easilr provided, as by drilling. Circumferentially disposed around the open end of the sleeve 50 is a bead 56 that bears against the internal surface of the casing 10. The end of the sleeve 50 that has the bead 56 formed thereon has an inclined or sloping cam surface 58. A coil spring 59 is held in a compressed state between the flange 26, which is fixed, and the shoulder 26; in this way, the sleeve is constantly biased to the left.

Of importance in practicing the instant invention is a plurality of resilient tines or spring fingers 60, these tines 60 being integral with a fixed sleeve labeled 62 which has a cylindrical bore 64. Although not all are visible, four such tines are embodied in the illustrated device; the actual number is not critical, however. Close attention is directed to the free ends 66 of the tines 60, for these free ends are enlarged and each is formed with a sloping cam follower surface 68 which bears against the above-mentioned cam surface 58. Inasmuch as the free ends 60 perform a camming action themselves, each end is formed with an inwardly disposed sloping cam surface 70. At this time, it should be pointed out that the tines 60 are heat treated so as to flex into the position shown in FIGURE 2. Thus, there is an inherent degree of resiliency that automatically urges the tines 60, when 3 free to do so, into the inwardly disposed position appearing in FIGURE 2.

However, a longitudinally shiftable plunger 72 having a beveled end thereon which forms an inclined or sloping cam follower surface 74 bears against the respective cam surfaces 70 so as to retain the tines 60 in the outwardly disposed position illustrated in FIGURE 1. The plunger 72 has an intermediate flange 76 of only slightly lesser diameter than the bore 64 of the sleeve 62 so that the plunger 72 is guided in a longitudinal direction.

The other end of the plunger 72 has been designated by the numeral 78 and abuts directly against a fusible pellet 80, such as indium, which becomes molten when subjected to enough heat, approximately 155 C. for this material. The pellet 80 is contained in a well 82 that is formed at this particular end of the casing and it will be discerned that there is a slight difference between the diameters of the plunger 72 and the well 82 so as to provide enough clearance for the molten indium or other fusible material.

The ease with which my thermal fuse can be assembled will now be described. First, the pellet 80 is placed in the well 82, which can be most readily accomplished with the casing 10 vertical. Access to the well 82 is via the opposite and now open end of the casing 10. Also through the open end of the casing can be inserted the plunger 72 and then the sleeve 62 with the resilient tines 60 thereon. Next, the sleeve or cup-shaped member 50 is placed within the casing 10. The coil spring 59 is then inserted so that one portion thereof encircles the guide 48. The bushing 22, the dielectric sleeve 34 and the tubular contact 36 will normally be pre-assernbled and then placed as a unit into the open end of the casing 10. After doing this, the solder 32 is applied and the bushing 22, together with its sleeve 34 and tubular contact 36, are held or anchored in a fixed relationship. The beveled entrance 40 on the tubular contact 36 allows the split ball contact 42 to enter the tubular contact 36, the slot 46 permitting the halves of the ball contact 42 to compress slightly and in this way good electrical engagement is made with the tubular contact 36.

Having presented the foregoing information, the manner in which my thermal fuse functions will now be briefly referred to. It will be appreciated that FIGURE 1 depicts the device in a condition ready for actual use. However, when the pellet 80 becomes sufficiently heated, it will become molten so as to no longer resist shifting or longitudinal movement of the plunger 72. It will be recalled that the tines 60 have an inherent resiliency which urges them into the position pictured in FIGURE 2, which applies the requisite amount of force so as to move the plunger 72 slightly in the direction of the well 82. The small amount of clearance between the wall of the well 82 and the cylindrical surface of the plunger 72 permits the now molten material to escape. It will be appreciated, though, that the pellet 80 does not have to reach a completely molten state in order to trigger the contact separation, for when enough material constituting the pellet 80 has been urged out of the well 82 via the annular clearance between the wall thereof and the plunger, then the plunger will shift enough so as to allow the tines 60 to contract or more inwardly to a position such that the sleeve 50 is no longer obstructed. When this occurs, the coil spring 59 immediately forces the sleeve 50 in a longitudinal direction to the left which quickly withdraws or separates the contact 42 from the contact 36. The expansion of the spring 59 is in such a direction a" to assist in the shifting of the plunger 72, this being through the agency of the sloping cam surface 58 which acts against the various surfaces 68 formed on the free ends of the tines 60, The bore 54 of the sleeve 4 50 serves as a recess or cavity that readily accommodates the tines 60 as the sleeve 50 moves to the left and into its position depicted in FIGURE 2. It will be understood that the ball contact 42 is completely withdrawn from the tubular contact 36, and the circuit is broken or interrupted that previously existed from the wire 14 through the casing 10, the sleeve 62 and its tines 60, the sleeve 50 and its shank 44, the contacts 36, 42 and-the wire 38.

I claim:

1. A thermal fuse comprising first and second cooperable contacts, a sleeve attached to said first contact so as to move said first contact away from said second contact when said sleeve is moved in one longitudinal direction, a coil spring biasing said sleeve in said one direction, at least one resilient tine fixedly anchored at one end, the free end of said tine being movable between an outwardly disposed position in obstructive engagement with one end of said sleeve to prevent longitudinal movement of said sleeve in said one direction and an inwardly disposed non-obstructive position with said sleeve to allow longitudinal movement of said sleeve in said one direction, the free end of said tine having an inwardly located sloping cam surface, a longitudinally shiftable plunger having a sloping cam follower surface adjacent one end in engagement with said sloping cam surface, and a fusible pellet which becomes molten when subjected to sufficient heat, said pellet engaging the other end of said plunger to prevent longitudinal movement of said plunger in said one direction and thus maintain said sloping surfaces in contact with each other and thereby to hold the free end of said tine in its said obstructive position with said sleeve, whereby when said pellet becomes sufliciently molten to allow said plunger to shift, the release of said sleeve is triggered by the inward movement of said tine so that said coil spring acts in a direction to urge said sleeve and said first contact relative to said second contact.

2. A thermal fuse in accordance with claim 1 in which said contacts are telescopically engaged, the release of said sleeve by the inward movement of said tine causing said first contact to be moved out of engagement with said second contact.

3. A thermal fuse in accordance with claim 1 in which said tine normally flexes inwardly to cause the free end of said tine to assume its inward non-obstructive position but is held in its outward obstructive position by said plunger.

4. A thermal fuse in accordance with claim 1 in which said one end of the sleeve is internally formed with a sloping cam surface and the free end of said time is formed with a sloping cam follower surface, whereby said sleeve acts in a direction to move the free end of said tine toward its said non-obstructive position.

5. A thermal fuse in accordance with claim 1 includ ing a plurality of similar resilient tines, said tines being integrally attached to a fixed sleeve member, and said plunger having an integral intermediate flange thereon slidably engaging said fixed sleeve.

References Cited UNITED STATES PATENTS 2,955,179 10/1960 Milton et al. 200-142 3,180,958 4/1965 Merrill 200-142 3,274,361 9/1966 Ebensteiner et al. 200-142 BERNARD A. GIL-HEANY, Primary Examiner. H. B, GILSON, Assistant Examiner. 

